1. Field of the Invention
The present invention provides a method for removing electrostatic charges from a clean room with a laminar flow apparatus, and more particularly, to a method for preventing electrostatic discharge in a clean room.
2. Description of the Prior Art
A modern clean room is not only able to remove the particles floating in the air quickly, but is also able to control the temperature, the humidity, the pressure, and to prevent electrostatic charges and electromagnetic interference. In recent years, electrostatic charge has become a serious problem, which all integrated circuit (IC) manufacturers cannot afford to ignore. The reason for this is because there is no regularity in the way the electrostatic charges are generated. In the manufacturing and assembling processes of electronic devices, the electronic devices are sometimes apt to be damaged at about 10V. If all electronic devices within a unit have completely broken down, the defect is easily detected by quality control. However, if the electronic devices are slightly damaged, or if only a portion of the electronic devices have completely broken down, it is very difficult to identify performance deterioration of such devices by routine testing. Broken or damaged devices are not always identified following processing, even upon the delivery to customers or when in use by customers. A quality control procedure is difficult to implement and hence the loss and the impact incurred are unpredictable.
The typical method for preventing electrostatic charges in a clean room is to utilize an ionization bar or an ionization system to neutralize electrostatic charges. In the former case, germanium or 100% tungsten is utilized as an emitter. By inputting compressed dry air (CDA) into the emitter and applying a voltage source to the emitter, charges are generated to neutralize the electrostatic charges. The number of emitters may be more than one, each of them is encapsulated by an isolative material to prevent short circuit and to fulfill safety requirements. It is more suitable for use with equipment than with a clean room.
The latter one is composed of a pair of parallel and spaced apart emitters and a top plate. The emitters are vertically disposed on the top plate, the top plate is fixed on the ceiling of the clean room, and the top plate is electrically connected to a system controller. By inputting compressed dry air into the pair of emitters and applying a positive voltage source and a negative voltage source to the pair of emitters, positive charges and negative charges, flowing downward in the same direction as the laminar flow, are generated to neutralize the electrostatic charges. Both the ionization bar and the ionization system are called ionizers (or ion generator) in accordance with their operational principle.
Since the top plate is electrically connected to the system controller, some settings and adjustments can be executed on the system controller, or be executed on a remote controller transmitter. In addition, a sensor or a monitor, for detecting the flow of positive ions and the flow of negative ions, and a corresponding electrical circuit system are frequently utilized. Based on practical requirements, the field strength of the positive and negative flows of ions must not be too strong or overly decayed, and the positive and negative flows of ions should remain at a predetermined balance.
In a clean room, many of the transports are by an automatic transportation system in order to avoid any probability of wafer contamination. With the various heights of destination equipment, the automatic transportation systems are likewise designed to have various heights.
Please refer to FIG. 1 of a schematic diagram of a method for preventing electrostatic charges 10 in a clean room according to the prior art. As shown in FIG. 1, the prior art method for preventing electrostatic charges 10 in a clean room is to dispose an ionization system A 14 and an ionization system B 16 on a ceiling 12 in the clean room (clean room not shown): The vertical distance between the output tips 22 of the emitters 18 of the ionization system A 14 and the ceiling 12 is approximately 84 cm. The vertical distance between the output tips 22 of the emitters 18 of the ionization system B 16 and the ceiling 12 is approximately 30 cm. The output tips 22 of the emitters 18 of the ionization system A 14 and the ionization System B 16 extend toward the floor 24 in the clean room. Compressed dry air (not shown) is input into the emitters 18.
An automatic transportation system A 26 and an automatic transportation system B 28 are disposed in the clean room. The two automatic transportation systems are over head transport (OHT) systems. The automatic transportation system A 26 is usually disposed in the inter-tunnel aisle (the aisle between tunnel and tunnel). The vertical distance between the automatic transportation system A 26 and the ceiling 12 is approximately 86 cm. The automatic transportation system B 28 is usually disposed in the tunnel in the clean room. The vertical distance between the automatic transportation system B 28 and the ceiling 12 is approximately 36 cm. All old type cart 32, pushed manually, is disposed on the floor 24 in the clean room. The height (the distance from the ceiling to the floor) of the clean room is approximately 3 m. The automatic transportation system A 26 supports a carrier 34, and the automatic transportation system B 2B suspends the carrier 34. The carrier 34 may be a wafer carrier, a reticle carrier, or other carrier. Therefore, both the automatic transportation system A 26 and the automatic transportation system B 28 may be a wafer transportation system, a reticle transportation system or other transportation system. The carrier 34, for transportating wafers or reticles, and other substances are also on the cart 32.
When a positive voltage source and a negative voltage source are applied to each pair of emitters 18, compressed dry air is input into each pair of emitters 18 to generate positive charges and negative charges through ionization. The positive charges and the negative charges flow downward, in the same direction as the laminar flow, to become a flow of positive ions 36 and a flow of negative ions 38. These ion flows 36 and 38 neutralize the electrostatic charges 10 on the carrier 34 in the automatic transportation system A 26, the automatic transportation system B 28, and the cart 32.
When the flow of positive ions 36 and the flow of negative ions 38 stream downward in the same direction as the laminar flow, a divergent angle 42 is created. The divergent angle 42 not only affects the range of area covered by the flow of positive ions 36 and the flow of negative ions 38, but also effects the rate at which the electric field strength degenerates with distance. The electric field strength (or equivalent ion density) is given by the following empirical equation:F=k(1/A)(1/Da)
where
F denotes the electric field strength,
k denotes a constant,
A donates the divergent angle between the flow of positive ions and the flow of negative ions,
D denotes the distance to the output tip of the emitter, and
a denotes an exponent of D.
When applying the prior art method to prevent electrostatic charges 10 in the clean room, the numerical values for the parameters in the above equation are: the divergent angle A is approximately 15 degree, the distance to the output tip of the emitter D is approximately 1.5˜2.0 m, and the exponent of D (a) is approximately 2. If the electric field strength at the output tip of the emitter is approximately 1 kV, the electric field strength of the flow of positive ions 36 and the flow of negative ions 38 generated by the ionization system A 14, when reaching the cart 32, is approximately 100˜150V according to the equation. Since the specification of the electric field strength in the protected area is typically 50˜150V when performing electrostatic charge protection in a clean room, the flow of positive ions 36 and the flow of negative ions 38, generated by the ionization system A 14, can neutralize the electrostatic charges 10 on the carrier 34 in the cart effectively. At the same time, materials carried in the carrier 34 in the cart 32 won't be damaged due to an over high electric field strength.
However as shown in FIG. 1, the ionization system A 14 cannot provide electrostatic charge protection to the carriers 34 in the automatic transportation system A 26 and the automatic transportation system B 28. Therefore, the carriers 34 in the automatic transportation system A 26 and in the automatic transportation system B 28 can only be protected by the flow of positive ions 36 and the flow of negative ions 38 generated by the ionization system B 16. The discharge amount of a general ionization system can only be adjusted slightly. Calculated with the same equation under the same conditions, the electric field strength of the flow of positive ions and the flow of negative ions, generated by the ionization system B 16, exceeds the upper limit of the specifications when reaching the carriers 34 in the automatic transportation system A 26 and the automatic transportation system B 28. The excessive electric field strength is due to the short distances between the output tips 22 of the emitters 18 and the automatic transportation system A 26 and the short distances between the output tips 22 of the emitters 18 and the automatic transportation system B 28. The electrostatic charges cannot be neutralized effectively and the materials carried in the carrier 34 of the automatic transportation system A 26 and the automatic transportation system B 28 may be damaged due to the excessive electric field strength.